I happen to be in the pressure washing business as well as the storage business and do graffiti removal. And you can't go wrong with this places products.

http://www.shopetsonline.com/Graffiti-Remover-s/38.htm

Taginator for masonry surfaces and tagaway for everything else. Also a graffiti remover that works on a truck will work on your metal buildings also.

But I highly recommend this stuff.

Randy's referral above look like a good source. Another good graffiti remover is available through paint stores, Home Depot, etc. under the Goof-Off brand. Go to www.goof-off.com, and locate the store nearest you by providing your zip code.

We use this on smooth surfaces, like metal, etc. We paint over graffiti on our walls.

If you want to go cheap HD has some stuff called Jasco Paint remover, but it will alter the color of the area you clean.

The stuff I recommend won't damage the surface if used properly and works on just about everything I have thrown it on.